Kiln-AI releases Harness Tuner, an eval-driven tuning loop for agent tools, prompts and models

Kiln-AI released Harness Tuner, an open-source CLI that tunes an agent's tools, prompts, skills, model and settings against the evals you already have. On one production multi-turn agent with over 200 tools, the team reports 2.5x fewer failures, 6x lower cost and 2x lower latency on held-out data with a smaller model. It runs on Claude Code, Codex or OpenCode subscriptions, keeps train, val and test splits enforced in code, and works with the free Kiln app for experiment tracking.